February 21, 2014

H10N8: A virus on the verge?

Does H10N8 pose a pandemic threat? The introduction of a new influenza A subtype into people is always a public health concern. However, pandemic viruses are characterised by high transmission. Sustained person-to-person transmission has not been reported with influenza A virus subtypes other than H1, H2, and H3 viruses, and so far H10 viruses are no exception. JX346 did not successfully spread to close contacts,3 and mild cases of H10N7 virus infection in Australia and Egypt did not transfer to exposed relatives.1

Experiments done to improve understanding of what is necessary for sustained transmission of avian H5N1 influenza A viruses in ferrets in laboratory settings10, 11 showed several mutations throughout the viral genome (mainly in haemagglutinin and the polymerase complex) that are needed for this adaptation. Of those, JX346 shows PB1 polymorphisms at positions 99 and 368, which are associated with enhanced replication and transmissibility in ferrets, and the well characterised mammalian adaptation PB2 627KLys.

However, despite more than 15 years of H5N1 transmission events from birds to people, none of these mutations resulted in a strain that could be transmitted between people.12

How virulent is the H10N8 virus? Although H10N8 is predicted to have low pathogenicity in poultry and other avian species, it is too early to say anything conclusive about its virulence in people because of the small number of cases. Even for avian influenza A H5N1 and H7N9 viruses, the real frequency of mild and asymptomatic infections is unknown, despite the many deaths associated with human infections, because diagnosis and detection is generally done only when patients are admitted to hospital, and therefore is biased towards severe cases.

While increased surveillance might also be responsible for the increase in number of human infections with avian viruses, most human infections are associated with avian viruses containing the H9N2 internal gene cassette, on the basis of available sequences.13, 14 Studies are needed to understand how this internal cassette helps avian influenza viruses seemingly well adapted to poultry to also jump more frequently into people and cause disease.

More human cases of avian influenza A H7N9 virus infection have been reported in China in the past year than with H5N1 viruses since their emergence in 1997.13, 14 Both reopening of live poultry markets and seasonality might have contributed to an apparent re-emergence of H7N9 human infections in the past month. Whether cases of avian influenza A H10N8 virus infection are going to increase is unknown, because how widely these viruses are circulating in poultry is unknown.

More surveillance will be needed to establish the origin of H10N8 and to monitor potential future transmission events. Additionally, other new avian influenza virus subtypes, reassorting with H9N2 viruses, might emerge in the near future and cause human infections.

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Does H10N8 pose a pandemic threat? The introduction of a new influenza A subtype into people is always a public health concern. However, pandemic viruses are characterised by high transmission. Sustained person-to-person transmission has not been reported with influenza A virus subtypes other than H1, H2, and H3 viruses, and so far H10 viruses are no exception. JX346 did not successfully spread to close contacts,3 and mild cases of H10N7 virus infection in Australia and Egypt did not transfer to exposed relatives.1

Experiments done to improve understanding of what is necessary for sustained transmission of avian H5N1 influenza A viruses in ferrets in laboratory settings10, 11 showed several mutations throughout the viral genome (mainly in haemagglutinin and the polymerase complex) that are needed for this adaptation. Of those, JX346 shows PB1 polymorphisms at positions 99 and 368, which are associated with enhanced replication and transmissibility in ferrets, and the well characterised mammalian adaptation PB2 627KLys.

However, despite more than 15 years of H5N1 transmission events from birds to people, none of these mutations resulted in a strain that could be transmitted between people.12

How virulent is the H10N8 virus? Although H10N8 is predicted to have low pathogenicity in poultry and other avian species, it is too early to say anything conclusive about its virulence in people because of the small number of cases. Even for avian influenza A H5N1 and H7N9 viruses, the real frequency of mild and asymptomatic infections is unknown, despite the many deaths associated with human infections, because diagnosis and detection is generally done only when patients are admitted to hospital, and therefore is biased towards severe cases.

While increased surveillance might also be responsible for the increase in number of human infections with avian viruses, most human infections are associated with avian viruses containing the H9N2 internal gene cassette, on the basis of available sequences.13, 14 Studies are needed to understand how this internal cassette helps avian influenza viruses seemingly well adapted to poultry to also jump more frequently into people and cause disease.

More human cases of avian influenza A H7N9 virus infection have been reported in China in the past year than with H5N1 viruses since their emergence in 1997.13, 14 Both reopening of live poultry markets and seasonality might have contributed to an apparent re-emergence of H7N9 human infections in the past month. Whether cases of avian influenza A H10N8 virus infection are going to increase is unknown, because how widely these viruses are circulating in poultry is unknown.

More surveillance will be needed to establish the origin of H10N8 and to monitor potential future transmission events. Additionally, other new avian influenza virus subtypes, reassorting with H9N2 viruses, might emerge in the near future and cause human infections.